Ask really nicely?
If you have been keeping your probation officer informed of your genuine economic hardship all along, you can discuss possible resolutions - a common one is to extend probation to allow for additional time to pay the fees. You won't know what your probation officer can or will do until you talk to him.
If your probation officer believes that you're able to pay but have preferred to spend your money on things other than your fees, you shouldn't expect much sympathy.

